Referring to FIG. 1 one type of smelter has a reaction shaft 10 into which feed material is inserted together with oxygen and the fluxing agents silica and limestone. The mixture ignites instantaneously to form hot sulphur dioxide gas and the lead, zinc, iron and other metals form metal oxides. The resulting semi-fused slag falls to the bottom of the first compartment along with the coarse coke. The dry feed is injected at the top of a reaction shaft of the smelter together with oxygen. The coke collects as a surface layer, called a “coke checker”, floating on top of the molten slag. When the metal oxides percolate through this layer of burning coke, they are reduced and the lead is converted to metal as bullion.
The bullion continues to settle through the molten slag layer beneath the coke checker. Together with the zinc-bearing iron slag, the bullion passes under a partition wall into a compartment, which is an electric furnace. This partition wall extends into the molten slag forcing the hot sulphur dioxide gas to pass through a waste heat boiler and onto an electrostatic precipitator rather than into the electric furnace compartment
The metallic slag containing ail of the iron and most of the zinc from a furnace is transferred in 70 tonne batches to a coal-fired fuming furnace 12. To recover the zinc, fine coal and air are injected one meter below the top of the slag bath. The heat generated causes the zinc to fume as a vapour from the furnace bath and is immediately reoxidized by tertiary air above the bath to form zinc oxide fume. These fumes and hot gases are cooled in a waste heat boiler 14 before passing through a baghouse to collect the zinc fumes for treatment in an adjacent Fume Leach Plant (not shown). The waste heat boiler 14, see FIG. 2, consists of a room having a plurality of closely spaced vertical pipes 16 against the surfaces 18. Water runs through these pipes 16 picking up heat from the gases inside and exiting as hot water or steam. In time deposits form over the exterior of the pipes, reducing their effectiveness in cooling the gases.